The present invention relates to electrical discharge machining methods and apparatus. More particularly, the invention relates to an improved electrical discharge machining method in which, while an electrical tool is moved relative to a workpiece in a primary machining direction, hereinafter referred to as "a Z-axis," the electrode tool is also moved in a plane perpendicular to the Z-axis, hereinafter referred to as "X-Y plane," to effect an electrical discharge machining operation between the electrode tool and the workpiece, that is, to machine the workpiece using electrical discharge, and to an apparatus for practicing the method.
An electrical discharge machining method in which a workpiece is machined with an electrode tool by a primary machining movement in the Z-axis direction and an auxiliary machining movement in the X-Y plane is well known in the art. Such a method is disclosed, for example, in Published Japanese patent application No. 3594/1966. The conventional method is advantageous in that a plurality of machining steps, namely, a coarse machining step, an intermediate machining step, an intermediate finish machining step, a finish machining step and a fine finish machining step can be continuously carried out with a single tool electrode.
In general, in the coarse machining steps, a primary machining movement in the Z-axis direction is utilized and the workpiece is machined with large current as a result of which the amount of material removed from the workpiece, that is, the "machining width" is relatively large. On the other hand, in the remaining machining steps up to the fine finish machining step, the discharge current is gradually decreased and the machining width is therefore decreased. In these machining steps, the above-described auxiliary machining movement in the X-Y plane makes the machined surface flat and smooth while correcting for the decrease of the machining width with the single electrode tool.
In a conventional electrical discharge machine having an auxiliary machining movement capability, chips of workpieces and denatured components of an insulating machining liquid which had been thermally decomposed by high temperature arcs during electrical discharge which may accumulate in a discharge gap can be removed by the pumping action of the machining liquid due to the auxiliary machining movement to therefore provide the machined surface with a suitable smoothness.
FIG. 1 shows a general machining state produced with the use of the conventional method in which a workpiece 12 is machined with an electrode having a scalene triangle section. An auxiliary machining movement in the X-Y plane is imparted to the electrode 10. That is, the electrode 10 is subjected to an orbital or circular motion. The radius of the circular motion is indicated by R in FIG. 1. This conventional method can provide the same effect as that in the case where an electrode larger by the radius R than the electrode 10 is used. However, the conventional method is still disadvantageous in the following points. As is apparent from FIG. 1, the corners are machined into arcs with a radius R. That is, as the machining configuration is extremely different from the configuration of the electrode 10, it is impossible with the conventional method to carry out electrical discharge machining operation with a very high accuracy.
A variety of auxiliary machining feeding methods have been proposed in order to eliminate the difficulties accompanying circular motion. One of these auxiliary machining feeding methods is illustrated in FIG. 2 in which an electrode 10 is displaced relative to a workpiece 12 equidistantly and radially towards the corners. In FIG. 2, the radial displacements towards the corners are indicated by vectors a, b and c each having a magnitude R. However, as is clear from FIG. 2, the machined configuration is considerably different from the configuration of the electrode 10 because of the resulting corner angles even if the radial relative displacements are carried out according to the conventional method. That is, it is still impossible for the conventional method to provide highly accurate electrical discharge machining.
Another conventional auxiliary machining feeding method is illustrated in FIG. 3 which is an improvement of the method of FIG. 2. In the method of FIG. 3, the sides A, B and C of a triangular electrode are moved relative to the workpiece with a similarity scale factor k. However, this method is still disadvantageous in that, except for the case where the electrode 10 is a regular triangle in section, the machining widths .alpha., .beta. and .gamma. between the workpiece 12 and the electrode 10 are different from one another and therefore the machining configuration produced does not correspond to the configuration of the electrode 10. In other words, in the conventional method of FIG. 3, the widths .alpha., .beta. and .gamma., which are enlarged because of the auxiliary machining movement from the actual configuration of the electrode, are different from one another depending on the configuration of the sides. Accordingly, uniformly machined surfaces cannot be obtained by a plurality of electric discharge machining steps from the coarse machining step to the fine machining steps. That is, the machined surfaces are not sufficiently high in surface flatness.
An improved electrical discharge machining method the invention will initially be described with reference to FIG. 4 which illustrates an auxiliary machining movement operation. In FIG. 4, the machining configuration produced by an electrode 10 is indicated by straight lines A', B' and C' which are parallel to the corresponding sides of the electrode 10 and are spaced by a distance R from the corresponding sides of the electrode 10. The straight lines A', B' and C' cross one another at points P.sub.1, P.sub.2 and P.sub.3, respectively. The electrode 10 has vertices q.sub.1, q.sub.2 and q.sub.3 and angles .theta..sub.1, .theta..sub.2 and .theta..sub.3.
Among auxiliary machining movement vectors a, b and c for moving the sides A, B and C of the electrode 10 outwardly by equal distances R, the vector a will be described by way of example. The lines A' and B' include the intersections r.sub.2 and r.sub.1 which are obtained by drawing perpendiculars of length R from the vertex q.sub.1 to the lines A' and B', respectively. The rectangle P.sub.1 r.sub.2 q.sub.1 r.sub.1 is a square. When the vertex q.sub.1 is displaced by the length of the vector a, the vertex q.sub.2 is moved to a point q.sub.2 '. The angle at the vertex q.sub.2 of the parallelogram P.sub.1 q.sub.1 q.sub.2 q.sub.2 ' is .theta..sub.1 /2. Therefore, the vector a has an azimuth of .theta..sub.2 +.theta..sub.1 /2 and a magnitude of R/sin (.theta..sub.1 /2). By similar calculations, the other displacement vectors b and c have azimuths and magnitudes as indicated in FIG. 5. If the relative displacement of the electrode 10 and the workpiece 12 is effected by auxiliary machining movement in the X-Y plane according to the vectors a, b and c, then a satisfactory machining configuration is obtained which is in conformance with the configuration of the electrode 10, specifically with the angular configuration corresponding to that of the electrode.
FIG. 6 shows continuous vectors, auxiliary machining movement vectors, which are obtained by conversion of the vectors a, b and c. In this case, a desired machining configuration can be obtained by providing relative displacements according to the vectors in FIG. 6 to the electrode 10 and the workpiece 12. However, it should be noted that, in practice, a considerably complex technique is required for implementing the above-described vector calculations and for controlling the locus of the electrode tool according to the vector calculation due to the following reasons. The configuration of the electrode 10 is not always limited to the above-described triangle and is often more intricate. In the case of an electrode of intricate configuration, the above-described vector calculation and locus control can be achieved only by a so-called "N/C control" technique which requires a great amount of complex calculation equipment and/or computer programs to implement.
In a practical electrical discharge machining operation, the discharge gap is 10 to 100 .mu.m and the electrode tends to be consumed unevenly in dependence upon its configuration. Therefore, the displacement vectors are determined by the angles, thickness and area of the electrode instead of solely by vector calculation. In most cases, the displacement vectors are determined graphically.
The invention further relates to an electrical discharge machining control device which operates to control the displacement of the relative position of an electrode and a workpiece so that the relative positions of the electrode and workpiece are changed not only in the main direction in which the workpiece is machined with the electrode but also in a plane perpendicular to the Z-axis without rotating the electrode while a predetermined machining discharge gap of typically 10 to 100 .mu.m is maintained between the electrode and the workpiece.
As disclosed in the above-mentioned Japanese patent application No. 3594/1966 as to relative displacement of the electrode and the workpiece in the X-Y plane, the electrode is moved along a revolving orbit or a star-shaped orbit (radial motion) so that the workpiece is machined to a larger or smaller size compared with the size of the electrode. The relative displacement value in these machining operations is considerably small, typically only 50 to 500 .mu.m, corresponding to a so-called "finish margin".
Recently, an N/C (numerical controller) device has been employed to provide such an orbit or locus as described above for the movement of the electrode and the workpiece wherein the workpiece is automatically machined by controlling the movement of a table on which the workpiece is mounted or the movement of a head on which the electrode is mounted in the X-Y plane. In such an N/C device, a desired locus is programmed on a paper tape in advance which is loaded into the N/C device. The coincidence of the programmed locus with the desired locus may be checked by moving the table without actually machining the workpiece. However, it is considerably difficult to visually confirm whether or not the programmed locus coincides with the desired locus because the amount of relative displacement is very small as described above. After the machining of the workpiece has been started, it is substantially impossible to confirm the locus. Accordingly, the acceptability of machining conditions can be determined only after the machining operation has been completed.
Accordingly, a further object of the invention is to provide an electrical discharge machining control device in which a small displacement locus is enlarged so as to be visually checked and two-dimensional movement in the X-Y plane of the electrode and workpiece is displayed on a display unit such as a cathode-ray tube.